The present invention relates to a temperature-sensitive fuse and more particularly to a temperature-sensitive fuse capable of interrupting or breaking an electric circuit when a predetermined ambient temperature is reached.
Conventionally, a temperature-sensitive fuse is known, which comprises a cylindrical, tubular, electrically-conductive casing with a first conductor electrically connected to one end of the casing, and a second conductor projecting into the other end of the casing in such a manner as to be electrically insulated therefrom; a temperature-sensitive, fusible pellet disposed in the end portion of the casing containing the first conductor; and an electrically conductive, movable member held between the fusible pellet and the inner top portion of the second conductor, which is normally in contact with the fusible pellet and the inner wall of the casing, thereby completing the electrical connection between the first conductor and the second conductor, and when a predetermined ambient temperature is reached and the pellet therefore melts and collapses, the movable member being moved away from the second conductor, thereby breaking the electrical connection between the first conductor and the second conductor, and interrupting the circuit.
In U.S. Pat. No. 3,781,737 and U.S. Pat. No. 3,924,218, there is disclosed a temperature-sensitive fuse of the above-mentioned type employing three metallic balls which serve as the above-mentioned movable member and which are disposed in contact with the top portion of the second conductor and the inner wall of the casing.
However, in a temperature-sensitive fuse employing the three metallic balls, unless all three metallic balls are disposed in uniform contact with the top portion of the second conductor and the inner wall of the casing, there is a risk that those balls may not work smoothly as the movable member when the fusible pellet melts and collapses. Therefore, in producing such temperature-sensitive fuses, it is required that those metallic balls be positioned accurately in uniform contact with the top portion of the second conductor and the inner wall of the casing. However, in practice, it is extremely difficult to produce these temperature-sensitive fuses, satisfying that requirement.
Further, when the fusible pellet is in the normal state of being solid and uncollapsed, the three metallic balls are in pressure contact with the top portion of the second conductor by the compression spring. If the force exerted on the balls by the compression spring is great, the balls may be moved outwards towards the surface of the tubular casing and positioned so as to cut into the surface of the inner wall of the casing. When the balls are in such a forced positioned, they do not move when the pellet melts and collapses.
Since the temperature-sensitive fuse is employed as an emergency switch, it is an indispensable requirement that it work without fail to interrupt an electric circuit under a predetermined condition and, therefore, any tendency of the above-mentioned movable members not to operate has to be avoided by all means.
Further, in U.S. Pat. No. 3,519,972, U.S. Pat. No. 3,778,742 and U.S. Pat. No. 4,001,754, there are disclosed temperature-sensitive fuses employing a flat disc as the movable member. In these temperature-sensitive fuses, a central portion of the disc is in contact with the top portion of the second conductor, while the other peripheral portion of the disc is in contact with the inner wall of the casing. In these temperature-sensitive fuses, an extremely thin, flat disc is employed, possibly for the following reason: Normally, the disc has to be in as close as possible contact with the inner wall of the casing for assuring electric current flow from the casing to the disc. Further, upon melting and collapsing of the fusible pellet, the disc is required to slide quickly along the inner wall of the casing, away from the second conductor.
These functions of the disc can be attained, for example, by designing the disc so as to be as thin as approximately 0.1 mm, increasing the resilience of the disc. However, such a thin disc will give rise to various problems in the temperature-sensitive fuse. For example, the thin disc has to be reinforced by attaching an additional disc thereto. In manufacturing the temperature-sensitive fuses whose monthly production rate is in the range of several hundred thousand to several million units, an additional part (the reinforcing disc), even if it is but a single additional part for each temperature-sensitive fuse, will have a significant effect on the production-line operation and cost.
Furthermore, great manufacturing care is required to dispose such a thin disc accurately in the right position in the casing. Specifically, the thin disc has to be positioned accurately normal to the second conductor. If the disc is inclined, sticking may occur and the secure and quick operation of the temperature-sensitive fuse cannot be guaranteed in an emergency.
Further, when the fusible pellet melts and collapses and the disc is moved away from the second conductor, since the peripheral portion of the disc is in pressure contact with the inner wall of the casing, and thus there is sliding friction between them, it could happen that the force exerted to move the disc against the molten pellet would be insufficient. When that happens, a sufficient space is not obtained between the disc and the second conductor, so that the breakdown voltage is reduced. Further, a comparatively long time is required before the disc is separated from the conductor due to the friction between the disc and the inner wall of the casing, resulting in the production of electric sparks between them and the disc being joined to the inner wall of the casing by the heat of the electric sparks, so that the interruption of the electric circuit is hindered.